Controlled data flow

ABSTRACT

A method for controlling MPEG compatible data reproduced in sectors by a digital video disk player. The method comprises the steps of transducing groups of sectors including requested sectors having MPEG compatible data required for processing, and unrequested sectors having MPEG compatible data not required for processing. Coupling the requested sectors exclusive of the unrequested sectors to a data processor for processing. Processing the requested data sectors to extract the required MPEG compatible data representative of video information.

This is a non-provisional application of provisional application Ser.No. 60/039,573 filed Feb. 18, 1997 by Mark A. Schultz et al.

FIELD OF THE INVENTION

This invention relates to the reproduction of a digitally encoded signalfrom a medium and in particular to the selection of reproduced data forsubsequent processing.

BACKGROUND OF THE INVENTION

The introduction of disks recorded with digitally compressed audio andvideo signals, for example, utilizing MPEG compression protocols, offersthe consumer sound and picture quality virtually indistinguishable fromthe original material. However, consumer users will expect such digitalvideo disks or DVDs to offer features similar to those of their analogvideo cassette recorder or VCR. For example, a VCR may reproduce ineither forward or reverse directions at speeds other than the recordedspeed. Such non-standard speed playback features are also known as trickplay modes. The provision of trick play features are less easilyprovided with MPEG encoded video signals due to the hierarchical natureof the compression which forms pictures into groups having varyingdegrees of compression. These groups are termed groups of pictures orGOPs, and require decoding in sequence. A detailed description of theMPEG 2 standard is published as ISO/IEC Standard 13818-2. However, insimple terms, an MPEG 2 signal stream may comprise three types ofpictures having varying degrees of content compression. An intra-codedframe or I frame has the least compression of the three types and may bedecoded without reference to any other frame. A predicted frame or Pframe is compressed with reference to a preceding I or P frame andachieves greater degree of compression than an intra-coded frame. Thethird type of MPEG frame, termed a bi-directionally coded or B frame,may be compressed based on predictions from preceding and/or succeedingframes. Bi-directionally coded frames have the greatest degree ofcompression. The three types of MPEG frames are arranged in groups ofpictures or GOPs. The GOP may for example contain 12 frames arranged asillustrated in FIG. 1A. Since only an intra-coded frame is decodablewithout reference to any other frame, each GOP may only be decodedfollowing the decoding of the I frame. The first predicted frame or Pframe, may be decoded and stored based on modification of the stored,preceding I frame. Subsequent P frames may be predicted from the storedpreceding P frame. The prediction of P frames is indicated in FIG. 1A bythe curved, solid arrow head lines. Finally, bi-directionally coded or Bframes may be decoded by means of predictions from preceding and orsucceeding frames, for example, stored I and P frames. Decoding of Bframes by predictions from adjacent stored frames is depicted in FIG. 1Aby the curved, dotted arrow head lines.

The hierarchical nature of the coded frames comprising MPEG groups ofpictures necessitates that the I and P frames of each GOP are decoded inthe forward direction. Thus, reverse mode features may be provided byeffectively jumping back to an earlier, or preceding I frame and thendecoding in a forward direction through that GOP. The decoded framesbeing stored in frame buffer memories for subsequent read out in reverseto achieve the desired reverse program sequence. FIG. 1B illustratesplay back in the forward direction at normal speed and at a time priorto time to, a reverse three times speed mode trick play mode isselected. The trick play mode is initiated at time t0 where I-frameI(25) is decoded and displayed. The next frame required for decoding isI-frame I(13), thus the transducer is repositioned, as indicated byarrow J1 to acquire frame I(13). Having recovered and decoded I-frameI(13), the transducer tracks, as indicated by arrow J2 to acquire anddecode frame P(16). The process is repeated as indicated by arrows J3,J4. Following the acquisition and decoding of frame P (22) thetransducer is moved as depicted by arrow Jn to recover frame I(1). Tosmoothly portray scene motion requires the decoding and display of I, P,and possibly B-frames. The jump and play process is repeated forpreceding GOP, thereby progressing haltingly backwards through therecords whilst smoothly portraying the program material in a reversesequence at the video output.

The transducer or opto-pickup is servo controlled to follow the recordedtrack and to maintain optical focus. In addition the transducer may berepositioned or jumped to a specific sector of the recorded trackresponsive to a sector address coupled to the transducer control servosystem. Such a transducer jumps may result from parental guidanceselection, alternative angle selection, user searching or trick modereproduction. During transducer repositioning the reproduced bitstreamwill disappear and the error correction buffer will contain gaps.However such gaps are of short duration and are flagged by a data validsignal. The transducer is quickly repositioned and refocuses to acquiredata from the recorded track, however, the recovered data may precedethat requested since it was transduced from sectors occurring possiblyone revolution before the wanted sector address. This acquisition ofunwanted data results as the disk rotates to position, or approximatelyposition, the wanted sector under the transducer. Thus, although thetransducer is repositioned, the error corrected bitstream 41 coupled tothe back end initially includes data from unwanted preceding sectorswhich must be identified by a microcontroller and discarded. Suchprocessing of unwanted replay data represents unnecessary, additionalmicrocontroller and buffer memory utilization.

SUMMARY OF THE INVENTION

In an inventive arrangement, unnecessary processing of unwanted sectordata is avoided. A method for controlling data reproduced in sectors bya disk player employing optical read out, comprises the steps oftransducing groups of sectors including sectors wanted for processing,and sectors unwanted for processing. Supplying the wanted sectorsexclusive of the unwanted sectors to a data processor for processing,and processing the wanted data sectors to extract data thereinrepresentative of video information.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an MPEG 2 group of pictures.

FIG. 1B illustrates recorded groups of pictures, during replay andreverse trick play at three times speed.

FIG. 2 is a block diagram of an exemplary digital video disk playerincluding inventive arrangements.

FIG. 3 is a block diagram showing in greater detail part of FIG. 2 anddepicting various inventive arrangements.

FIG. 4 is a block diagram depicting inventive arrangements in additionto those of FIG. 2.

FIG. 5A depicts an exemplary bit stream before track buffering.

FIGS. 5B-5C depict exemplary data in buffer memory.

DETAILED DESCRIPTION

FIG. 2 depicts an exemplary block diagram of a digital video diskplayer. In block 10 a deck is shown which may accept a digitallyrecorded disk 14 for rotation by a motor 12. A digital signal isrecorded on disk 14 as a spiral track containing pits with respectivepit lengths determined by an 8/16 modulation coding responsive torespective signal data bits. The record on disk 14 is read by pick up 15which gathers reflected illumination from a laser. The reflected laserlight is collected by a photo detector or opto pick-up device. Animaging device, for example a lens or mirror, which form part of pick-up15, is servo controlled and driven by motor 11 to follow the recordedtrack. Different parts of the recording may be accessed by rapidlyrepositioning the imaging device. Servo controlled motors 11 and 12 aredriven by integrated circuit drive amplifier 20. Pick up 15 is coupledto an opto preamplifier, block 30, which includes drive circuitry forthe laser illuminator and a preamplifier which provides amplificationand equalization for the reflected signal output from the opto pick-updevice. The amplified and equalized replay signal from opto preamplifier30 is connected to a channel processor block 40 where the replay signalis employed to synchronize a phase locked loop which is utilized todemodulate the 8/16 modulation employed for recording.

The MPEG encoded bitstream is encoded for error correction corrected bymeans of Reed Solomon product coding which is applied in blocks of 16sectors, where each sector contains 2048 bytes of data. Thus following8:16 demodulation the replay data stream is de-interleaved or unshuffledand error corrected by means of Reed Solomon product correctionimplemented in ECC buffer memories 45, and 46 of FIG. 4. Each bufferstores 16 sectors of the replay data stream arranged as an array tofacilitate de-interleaving and to enable the required row and columnproduct processing. The cascaded ECC buffer memories introduce a delayto reproduced serial bit stream which is approximately calculated by(2*16*1.4) milliseconds, where 2 represents the pair of ECC buffers, 16represents the number of sectors over which the correction is appliedand 1.4 milliseconds represents a sector period at 1× rotational speed.Thus the reproduced serial bit stream is delayed by a minimum ofapproximately 45 milliseconds.

The error corrected signal bitstream 41 is coupled via a link processorto a bit stream or mechanical/track buffer memory 60A. The track buffercomprises a DRAM memory type and is used to store an amount of replayeddata such that data losses during transducer or pickup 15 repositioningwill not result in any visible deficiency when decoded. Thus the finaloutput image stream will appear to be continuous or seamless to theviewer. Bitstream buffer memory 60A is part of an exemplary 16 megabitDRAM memory. A further exemplary 16 megabit SDRAM memory block ispartitioned to provide frame buffers 60C and 60D which provide storagefor at least two decoded image frames, compressed video bit streamstorage prior to decoding in buffer 60B, an audio bit stream buffer 60Eand other storage in buffer 60F. The channel processor 40 also includestiming control circuitry which control writing by link 505 to bitstreambuffer 60A. Data may be intermittently written to the bitstream bufferas a consequence of changes in replay track addresses, for example,resulting from user defined replay video content such as a “Directorscut”, parental guidance selection, or even user selectable alternativeshot angles. To facilitate more rapid access and recovery of therecorded signal, disk 14 may be rotated at an increased speed resultingin the transduced bitstream having a higher bit rate, and possiblyintermittent delivery. This higher speed, bursty bitstream may beeffectively smoothed by writing the intermittent bitstream to buffer 60Aand reading out for subsequent processing and MPEG decoding at a lower,more constant rate.

As has been described, the recorded data stream is arranged in ECCblocks of 16 sectors. Each sector has a unique sector identificationaddress which is protected with error correction bits and these areprocessed at ECC block 47 of FIG. 4. However, because the sector addressis short and sector specific, error correction processing by block 47introduces only an insignificant delay to sector address signal 42 (ofFIG. 4) which is coupled to provide position information to servocontrol integrated circuit 50. Integrated circuit 50 provides drive andcontrol signals for servo motors 11 and 12. Motor 12 rotates disk 14 andprovides servo controlled rotation at a plurality of speeds. The optopickup or transducer 15 is positioned and servo controlled by motor 11responsive to sector address signal 42, and in addition, may becontrolled to rapidly reposition or jump to another sector address, orlocation on the disk surface in response to a sector address request,transmitted by I²C control bus 514, and illustrated at element 54 ofFIG. 4. The digital video disk player is controlled by a centralprocessing unit or CPU, element 510 of block 500, which accepts thereproduced bitstream and error flags from channel IC 40, and providescontrol instructions to servo IC 50. In addition CPU 510 accepts usercontrol commands from user interface 90, and MPEG decoder controlfunctions from the MPEG decoder element 530 of block 500. A systembuffer memory 80 is addressed by and provides data to CPU 510. Forexample, buffer 80 may comprise both RAM and PROM memory locations. TheRAM may be used to store various data extracted from bitstream 41 by CPU510 for example such data may include descrambling or decryptioninformation, bitstream and frame buffer memory management data, andnavigation data. The PROM may, for example contain, transducer jumpalgorithms which facilitate trick mode operation at a selection ofspeeds in forward or reverse directions.

The MPEG encoded bitstream is coupled to link processor 505 in FIG. 3,which may function as a hardware demultiplexor to separate audio, videoand control information from the encoded bitstream. Alternatively,bitstream demultiplexing may be accomplished by software control ofdirect memory access, DMA of buffer 60A, from CPU 510 of FIG. 3. Theencoded bitstream in track buffer 60A is searched by microcontroller 510to locate and read headers and to extract navigation data.

Microcontroller 510 is coupled the front end via I²C control bus signal514 to control or request transducer repositioning to acquire the nextsector required by a trick play sequence. The transducer positioning maybe controlled by an advantageous stored sequence, or jump play patternwhich is indexed with reference to replayed sector addresses and GOPsector addresses read from the navigation pack data at the start of eachvideo object unit or VOBU. Exemplary sector addresses and VOBUnavigation pack are depicted in FIG. 5A. However, following transducerrepositioning, the sectors initially retrieved from the front end may beidentified by exemplary microcontroller 510 as not those requested bythe jump instruction. Thus, microcontroller 510 advantageouslyoverwrites this unwanted data in track buffer 60A and ensures that onlythe desired MPEG picture data is present in the buffer.

Having identified sector addresses or headers, microcontroller 510controls direct memory access of buffer 60A which effectively separatesMPEG data from other DVD formatted data stored in the buffer. Thus,video DMA 515 separates compressed video bits which are coupled forstorage in exemplary video bit buffer 60B. Similarly compressed audiobits are read from buffer 60A and stored in audio buffer 60E.Sub-picture data is also retrieved from track buffer 60A by DMA andstored in buffer 60F.

The compressed video bit stream in video bit buffer 60B is searched tolocate picture or higher level start codes by start code detector 520. Adetected start code signal 512 is coupled to microcontroller 510 whichthen communicates with MPEG decoder 530, via signal 511, to indicate thenext picture type, the quantizer setting and to initiate decoding. Adecoder status signal 513 is coupled back to microcontroller 510 toindicate completion of decoding and picture data available for displayor storage. Compressed video bit buffer 60B may be considered tofunction as a FIFO or circular buffer where the stored bitstream issequentially accessed for MPEG decoding, however, trick mode operationmay be advantageously facilitated by random access of buffer 60B, aswill be described.

Within MPEG decoder 530 the video bit stream is processed by a variablelength decoder 531 which searches the bitstream to locate slice andmacro-block start codes. Certain decoded pictures from each group ofpictures are written to frame buffers 60C and 60D for subsequent use aspredictors when deriving or constructing other pictures, for example Pand B pictures, of the GOP. Frame buffers 60C and 60D have a storagecapacity of at least two video frames. Separated audio packets arestored in audio bit buffer 60E which is read out and coupled for audiodecoding in block 110. Following MPEG or AC3 audio decoding a digitizedaudio signal results which is coupled to an audio post processor 130 fordigital to analog conversion and generation of various base band audiosignal outputs. A digital video output signal is reconstructed indisplay buffer 580 from decoded blocks read from reference frame buffer60C/D. However, during trick mode operation the output signal source maybe an advantageous field memory thus block processing within displaybuffer 580 may be controlled responsive to trick mode operation. Thedisplay buffer is coupled to encoder 590 which provides digital toanalog conversion and generates baseband video components and encodedvideo signals.

Operation of the exemplary video player illustrated in FIG. 2 may beconsidered with reference to FIG. 1B which illustrates a forward playand reverse trick play sequence. As described previously, the codedrelationship existing within each GOP necessitates that each group ofpictures is decoded in a forward direction starting from an I-frame orpicture. Thus, reverse mode features may be provided by effectivelyjumping back to transduce an earlier, or preceding I picture and thendecoding in a forward direction through that GOP. The decoded picturesare stored in frame buffer memories for subsequent read out in reverseorder. However, sequences that include B pictures may utilize furtheradvantageous features which will be described. In FIG. 1B it will beassumed that at some time prior to time t0, for example at I-pictureI(1), the exemplary video player assumed a forward play condition inresponse to a user command. Each group of pictures is decoded in theforward direction as illustrated in FIG. 1A by the arrow headed lineslinking I, B and P frames. At a time prior to time t0, a three timesplay speed reverse trick mode is selected, and initiated at time t0where I-picture 1(25) is decoded and displayed. As previously describedthe next picture required for 35 reverse trick play decoding isI-picture I(13), thus the transducer is moved, as indicated by arrow J1to acquire picture I(13). The signal recovery and decoding then followsa play sequence indicated in FIG. 1B by arrows J1, to acquire I(13), J2,to acquire P(16), J3, to P(19), J4 to P(22) . . . Jn. The intervening Bpictures shown in FIG. 1B are transduced but may be discarded asrequired by each specific trick play mode. To avoid the previouslydescribed requirement for additional reverse mode video buffering,various advantageous methods for MPEG decoder control and buffer controland allocation are employed.

In a first advantageous arrangement the storage capacity video bitbuffer 60B is effectively increased by selecting for storage onlypicture data that is to be used subsequently, for example, in anexemplary trick play mode B frames are not decoded, hence need not bestored in a video bit buffer. Thus only needed pictures are stored, andunwanted, or non-decoding picture data is discarded. To facilitate thisadvantageous selection between wanted and unwanted pictures requiresthat the video packet stream be pre-processed or searched to locate agroup_of_picture_header prior to storage in buffer 60B and MPEGdecoding. Thus pre-processing of the compressed video packet streamallows the determination of parameters such as, time_code, closed_gop,and broken_link data for each group of pictures or GOP. In addition, bypre-processing the video packet stream the picture_start_code may belocated thus permitting processing of the picture_header which in turnallows the determination of, for example, the temporal_reference,picture_coding_type (I, P and B). As a consequence of obtaining thesedata, picture size may be calculated thus permitting dynamic control ofmemory management virtually concurrent with the header processing.However, because the DVD format partitions MPEG like data into sectorsof 2048 bytes, and the video stream start codes (4 bytes) are not sectoraligned start codes may be distributed across a sector boundary. Adistributed start code is depicted in FIG. 5B, where a start code forpicture C is initiated at byte 2046 of sector 12 and is continued insector 13. Hence part of a start code may be in one video sector withthe remainder in the next video sector. As a consequence, anadvantageous bitstream searching method contends with a distributedstart code by identifying and saving a partial start code and setting aflag to indicate the occurrence. In the next video sector the remainderof the start code is recovered and the partial start code is completed.However, the video sectors containing the distributed start code may beseparated by other sectors containing, for example, audio, sub-picturesetc. In this situation start codes and payload data identified as fromintervening non-video sectors are discarded responsive to a set partialstart code flag. Thus with the occurrence of the next video sector, theremainder of the start code is recovered and the partial start code iscompleted.

The determination of picture data may be performed in units of sectorsreferenced in track buffer 60A. However, since a picture start code isnot constrained to start coincident with a sector boundary the resultinglocation of video sectors in units of sectors may inevitably includefragments of a preceding, possibly non-video sector. Determination orlocation of picture data/video sectors in units of sectors isillustrated in FIG. 5B where a start code for exemplary picture A isshown in sector 2 with the start code of next picture B, occurring insector 9. Thus equation 1 shows picture data location by sector count.Picture A starts in sector 2 and ends in sector 9, and has a duration of8 sectors. Unwanted data fragments are illustrated FIG. 5B, where videodata is referenced to (video) sector numbers, which may be directlyrelated to the sector number or address in the reproduced bit stream. InFIG. 5B an exemplary picture A is depicted with a picture start codeinitiated at byte 1000 of video sector 2. Clearly the preceding 999bytes of sector 2 correspond to data from a preceding picture. It ispossible to employ more detailed processing where the picture data islocated the units of bytes. Byte accurate processing may require morecomplexity of memory control than that required for sector levelaccuracy. However, if byte accurate processing is employed only completepicture data are stored in the video bit buffer, thus fragments areeliminated and hang up of MPEG decoder 530 is avoided. Byte accuratepicture determination is shown in FIG. 5B for exemplary picture A, wherea picture start code starts at byte 1000 of video sector 2 and picture Bstart code starts at byte 500 of sector 9. The size of picture A may becalculated in bytes by use of equation 2.

Having byte accurate picture addresses may allow microprocessor 510 topoint to a specific byte in the video bit buffer 60B from which to startdecoding by variable length decoder VLD 531 of FIG. 3.

If picture data is determined in units of sectors, the MPEG decoderreading pictures from the video bit buffer must be protected from hangup due to fragments of discarded pictures occurring before or after thewanted picture is decoded. Such picture fragments are depicted inexemplary video bit buffer of FIG. 5C which shows multiple sectorscontaining P and B pictures where unwanted data from a previous, orfollowing picture is shown with diagonal shading. Each video objectblock unit or VOBU includes navigation data that identifies the endsector address of the first I picture and the last sector addresses oftwo following reference or P pictures of the first GOP of the VOBU. Inaddition the navigation data includes sector addresses of I-pictures inpreceding and succeeding VOBUs, hence an I-picture only trick mode maybe all readily provided. However, problems resulting from picturefragments may be avoided if the end byte of the wanted picture can beidentified. Microprocessor 510/A, for example type ST20, is configuredas a hardware search engine which searches through the stored data tolocate the ending byte of the I-picture within the ending sector storedin track buffer 60A. Thus by identifying an I-picture, it alone may beloaded into video bit buffer 60B, hence avoiding the storage partialpictures which may cause problems of decoder lockup. The exemplarymicroprocessor 510/A may b e employed to find start codes in anI-picture only mode since the ending sector is known from the navigationdata. However, for P, B or multiple I-pictures of the VOBU, theexemplary microprocessor may not provide a practical solution sincetesting has to be performed on every byte of data in the bitstream,which represents an operationally intensive usage of microprocessor 510.

The location and determination of start codes prior to picture decodingmay be facilitated by an arrangement which utilizes the link interfaceblock 505 of FIG. 3 to search for start codes in the bitstream prior tobuffer 60A. Such use of link interface 505 advantageously provides earlypre-processing of picture headers which may be signaled tomicroprocessor 510. Thus, having identified picture headers, pictureswanted by a specific trick mode may be stored in exemplary track buffer60A and unwanted pictures being deleted by overwriting in the buffer.

In a second arrangement, start codes are located by use of Start CodeDetector 520 to search the bit stream in either the mechanical/trackbuffer 60A or the video bit buffer 60B. Although this method has anadvantage in start code detector design is known, the data must enterthe video bit buffer prior to initiating start code detection because ofthe MPEG bitstream requirement for contiguous data. Thus searchingwithin the mechanical/track buffer may be difficult to facilitate. Suchsearching may not optimally use memory, and exemplary microprocessor 510may be heavily loaded with interrupts, requiring the addition of asecond exemplary microprocessor 510A specifically to implement startcode detection.

In a further advantageous arrangement, start code detection isfacilitated by a second start code detector which searches the bitstream in track buffer 60A exclusively for start codes, thusadvantageously providing early pre-processing of picture headers inanticipation of processing and memory manipulation specific to trickplay operation.

Various methods have been described for the location of specificpictures in terms of their disk sector address and buffer locations,however the facilitation of visually smooth trick modes clearly requirestimely disk replay and specific picture access from memory. Althoughnavigation pack data provides picture access points on the disk, theseare limited in number within each VOBU. Hence to achieve temporallysmooth trick modes at multiple speeds requires the formulation of alocator table where picture type is referenced to its on disk sectoraddress and start code buffer location and address. The exemplarymicroprocessor 510/A may be employed to assemble the picture locatortable. The use of the picture locator table permits wanted pictureacquisition and manipulation.

The processing of the video packet stream prior to the video bit buffer60B may be advantageously employed for trick mode operation. Forexample, at a trick play speed or in a reverse replay mode, suchpre-processing permits trick play specific selection between pictures tobe buffered for decoding, and those unwanted pictures to be discardedbefore decoding. Such picture selection, for example discardingB-frames, may approximately double the number of I and P pictures storedin video bit buffer 60B during trick play operation. Thus by selectionand deletion, video bit buffer 60B stores only wanted, or trick playspecific pictures, hence more video object units or VOBUs may be storedfacilitating enhanced trick play operation.

It is advantageous to control MPEG picture decoding order based onknowledge of where the pictures start and stop in the video bit buffer.Thus knowledge of picture location in the video bit buffer 60B allowsmemory start pointers in the start code detector 520 and variable lengthdetector 531 to be directed to effectively randomly access pictures asrequired, for example, during trick mode operation. Operation inreverse, at play speed and or slow motion playback requires thereproduction of B-frames. Such reverse mode operation may beadvantageously simplified in terms of buffer memory requirements byreversing the order in which adjacent B pictures are decoded. Thisreversal of decoding order is achieved by setting the memory startpointers to enable decoding of the picture required by the trick mode.In addition buffer memory size and control may be simplified duringtrick play operation by advantageously skipping or not reading picturesin the video bit buffer as required by specific trick play algorithms.Trick play buffer memory size and control may be advantageouslyoptimized by enabling multiple decoding of pictures either immediatelyor as specifically required by the trick play algorithm. Thefacilitation of these advantageous features requires careful control ofread/write functions and the synchronization therebetween.

The block diagram of FIG. 4 shows the same functions and elementnumbering as those depicted in FIG. 2, however, FIG. 4 includesadditional inventive arrangements which will be explained.

The exemplary digital video disk player shown in FIGS. 2, 3 and 4 may beconsidered to comprise two parts namely a front end and a back end. Thefront end controls the disk and transducer with the back end providingMPEG decoding and overall control. Such functional partitioning mayrepresent an obvious solution for consistent, steady state, MPEGdecoding. However, with such partitioning of processing and control atthe back end the microcontroller may become overloaded, for example,during trick mode operation and in particular when playing in thereverse direction.

As has been described, microcontroller 510 is required to manage theincoming bitstream 41 received from the front end and identify wantedfrom unwanted data. In a first advantageous arrangement bitstream 41 iscontrollably coupled between the front and back ends. In the exemplaryplayer of FIG. 2 opto-pickup or transducer 15 may repositioned, as hasbeen described. Sector addresses derived in the back end are sent via anI²C control bus 514 to the front end servo system 50 to repositiontransducer 15. However, the opto-pickup or transducer 15 is servocontrolled responsive to a sector address which is truncated to removethe least significant digit. This address truncation allows acquisitionof sectors in groups or blocks of 16 sectors. This grouping is requiredto facilitate error correction (ECC) by means of Reed Solomon productcoding and payload data interleaving applied over 16 sectors duringrecording. Thus information is acquired from the disk in ECC groups of16 sectors, and in general, the retrieved data containing the wantedsector address is in advance, or preceding that requested by the backend processing. In addition, the transducer moves relative to therotating disk with either radial or tangential motion to acquire thetrack containing the EEC block of sectors within which the wanted sectoraddress or addresses reside. Thus, following repositioning, thetransducer refocusses and sectors are transduced as the disk rotatestowards the ECC sector block containing the requested or wanted sectorsaddress. Hence, if worst case positioning of transducer and wantedsector address are considered, many hundreds of unwanted sectors may betransduced. The since the number of sectors increases with increasingdisk radius, so too will the number of unwanted sectors reproduced. Inaddition acquisition of an earlier or preceding address may possiblyrequire a complete disk revolution with resulting unwanted sectorreproduction. Thus very significant amounts of unwanted data areproduced prior to the occurrence of the wanted sector address. This bitstream is depicted in FIG. 4 as signal 44, and contains both wanted andunwanted data which is coupled for error correction at BCC blocks 45 and46. The error corrected bitstream is output from ECC processing assignal 41 which is coupled to the back end where microcontroller 510identifies wanted from unwanted data.

An inventive arrangement is shown in FIG. 4, where data signal 44 outputfrom an 8:16 code demodulator and is coupled via a control element 45A,for example a transmission gate, or logic function, to Reed Solomonerror correction blocks 45 and 46. Control element 45A is controlled byelement 43 which functions to compare the recovered, current replaysector address, error corrected in block 47 and output as address signal42, with a sector address 53A, derived from the back end, whichrepresents the next wanted data, for example picture type. Thecomparison may be facilitated by a comparitor or logical function. Thuswhen the replay sector address 42 equals address 53A requested by theback end, the demodulated data output is enabled by signal 43A forcoupling to error correction buffer blocks ECC 45 and 46. Since errorcorrection is applied to groups of 16 sectors, the comparison ofrequested address with actual address is performed such that the ECCblock of sectors containing the wanted sector is enabled for ReedSolomon correction. For example, sector address comparison may befacilitated with addresses having a least significant bit truncated.

Since, for example, a B type MPEG picture may occupy 3 sectors where asan I type MPEG picture may require 30 sectors or more, the requestedsector address represents the initial data sector of a wanted picturetype. In addition signal 43A, which represents substantial equalitybetween wanted and replay sector addresses, may be considered torepresent a latch function where the logical state is maintained untilthe wanted address is changed i.e. until a further transducer jump isrequested. The receipt of a new sector address changes the state ofsignal 43A, which inhibits reproduced data until the new wanted addressoccurs in the replay signal and is detected by comparitor 43. Stateddifferently, signal 44 remains enabled for error correction, FCC blocks45 and 46 are enabled and output signal 41 is sustained, or in simpleterms, the disk continues to play until a different transducer positionis requested.

The detected replay occurrence of the wanted sector may be performed bycomparison with truncated sector addresses to ensure that errorcorrection buffers 45 and 46 are filled with the number of sectorsrequired for RS correction. In a further embodiment, the same detectedreplay occurrence may be employed using signal 45B to control or enableoperation of error correction buffer memory 45 and 46. In an alternativeinventive arrangement only the requested sector is enabled via outputcontrol element 46A. Selection by element 46A is different from thecontrol provided by elements 45A and 45B which, because of theinterleaved, or shuffled data format enable the ECC block containing therequested sector. Detection of the wanted replay sector may be performedby comparison of the actual replay sector address and the requested orwanted address. However, because this control function is performedessentially following error correction and de-shuffling which utilizebuffer memory, the resultant output signal 41 is delayed by at least oneECC block time period. Hence error corrected output data corresponds togroups of sectors transduced in advance of the wanted data (address)identified as present at the ECC buffer input. Clearly since the bufferdelay is known it may be compensated for in the control coupling ofsignal 43A to element 46A, for example by use of a delaying methoddepicted as t. Control element 46A is depicted as a series switchelement capable of enabling or disabling bitstream supply to the backend. Thus signal 43A, suitably timed to compensate processing and bufferdelays, may be applied to selectively enable de-interleaving bitstream41 for transmission to processing block 500. The use of the precedinginventive embodiments permits only transduced data from requestedsectors to be coupled to the back end for storage and decoding, thusreducing microcontroller 510 work load.

What is claimed is:
 1. A digital disk player for reproducing sectorscontaining MPEG compatible data, comprising: a transducer fortransducing from a disk, groups of sectors including a requested addresssector having MPEG compatible data required for processing andunrequested sector addresses having MPEG compatible data not requiredfor processing; a selector coupled to said transducer for selecting saidrequested address sector having MPEG compatible data exclusive of saidunrequested sector addresses, wherein the transducer and selector form afront end; a memory coupled to said selector for storing only saidrequested address sector; and, a decoder forming a back end for decodingsaid requested address sector.